Archaeologists can figure out how old a substance is by radiocarbon dating, but to do that they need to know what the substance is -- and that's not always clear. Radioactive material comes to the rescue again!
Scientists have discovered or manufactured quite a few new elements by hurling neutrons at old elements. Along the way they learned some things. One of the main things they learned is that there are two major ways a neutron of just the right energy can affect an element. First, the neutron can gift the nucleus of the element with some energy, which the element immediately re-gifts to the world in the form of a gamma ray. That's not the end of the process. With the addition of a neutron, the nucleus is unstable -- or radioactive. Sooner or later it will decay, maybe once, maybe several times. Each time it does so it will shoot out more gamma rays, and often a beta particle such as a positron or electron as well.
The timing of each of these events depends on the properties of the nucleus. Some elements will shoot out a gamma ray once and be done with it. Other nuclei will shoot out the gamma ray and then, depending on the half-life of the nucleus, send out another gamma ray in a few seconds, a few minutes, or another year.
Even if the half-life of an element is a few years, if you shoot hundreds of thousands of neutrons into hundreds of thousands of atoms, a few will decay right away. It's only the rate of decay that varies, and that's the give-away. Scientists know the timing and the number of gamma rays given off by each element, and each isotope of each element. To identify the chemical composition of a substance, they bombard it with neutrons and then take note of when and how the newly radioactive substance decays. This is called neutron activation analysis, and its a boon to archaeologists, who often find artifacts but rarely find any record of how the artifacts were made. A tiny sample of the substance, and a whole lot of neutrons, and they can pin down the chemical composition of their discovery.